1. Field of the Invention
This invention pertains to the storage or backup of data on rotating media, and particularly to storage of data using a removable disk drive.
2. Related Art and Other Considerations
Data, such as computer-generated data, is typically stored on a backup media for e.g., security and archival purposes. In view of reliability and affordability, magnetic tape has traditionally been a preferred media for data backup and restoration. Numerous types of magnetic tape drives are currently marketed, including helical scan tape drives and serpentine-type tape drives.
Another type of storage media is the rotating disk. Rotating disk media can be either magnetic or optical. Magnetic disks are traditionally classified as either floppy disks or hard disks. Both floppy and hard disks are handled by respective types of disk drives. Of the two types, the hard disk is normally employed for on-line data storage and has a significantly greater storage capacity. Moreover, in contrast to floppy disks, historically hard disks have been essentially permanently housed in the drive and removable or manipulated only by a technician.
In recent years removable disk drives have become available. A removable disk drive is a variation of the hard disk drive which allows a disk cartridge to be removed from the drive. Like the conventional hard disk, the disk of the removable disk drive is preformatted, as explained below.
Disk drives, both magnetic and optical, employ a format which divides the data into fixed-sized sectors. In a magnetic disk drive, typically a sector has either 512 or 1024 bytes of user data. Each sector is a self-contained unit that includes, along with the user data, certain control information such as synchronization marks, headers, cyclical redundancy check (CRC) characters, and error correction characters (ECC). Consequentially, the error correction characters (ECC) information contained on a disk is useful only for correcting errors within the sector in which it is stored. Such disk ECC information covers only the user data and the header of the sector. Other control information in the sector is not protected. If errors occur at locations which corrupt the control information, it is possible that the entire sector may not be readable. In other words, the disk ECC is to no avail for correcting information such as control information on disk.
In a normal hard disk drive the disk is sealed. Because of the particular manufacturing process for hard disks, defects on the disk which might cause errors are typically not a significant problem. In particular, in the manufacturing process for a hard disk, the disk manufacturer checks each sector and determines which sectors may be defective. If a sector is deemed defective, the sector is noted on a defect list for the disk. Sectors on the defect list are not available for recording data, i.e., remain unused. Then, after the disk is sealed in the disk drive, new defects on the disk are not introduced. The disk drive ECC thus can be minimal, and is designed only to correct small intermittent errors which can occur due to electromagnetic interference, radio frequency interference, environmental drift, and the like.
As mentioned above, in a removable disk drive the media is not sealed. Rather, the media is enclosed in a cartridge of some type, but the cartridge must be opened by the drive in order for heads of the drive to access the media. The process of opening and closing the disk cartridge can allow contaminates to enter the media and create new defects. In this case, the conventional drive ECC is insufficient to guarantee reliability of the data recorded on the removable disk.
Removable disk drives have numerous features appropriate for operations such as data backup and restore. Such features include long life, relative immunity to environmental limitations, high data rates, random access to data, and simple drive mechanisms. In addition, removable disk drives may ultimately be economically reasonable alternatives to other magnetic media. However, unless overcome, the inability to provide sufficient data integrity would jeopardize the usefulness of removable tape drives from integrity premium operations such as data back up and restore.
What is needed, therefore, and an object of the present invention, is method and apparatus for enhancing data integrity for removable disk drives.
A tape drive emulator is between a host computer and a removable disk drive and appears to the host computer as a sequential storage system, e.g., a tape drive. However, the tape drive emulator processes the data obtained from or applied to the host computer so that the data can be communicated to the removable disk drive in a manner compatible with a conventional removable disk drive. Yet in so doing, the tape drive emulator imposes on the data (1) a data organization, imperceptible to the disk drive, for rendering the data expressable and locatable in a tape drive format, and (2) an additional degree of error correction that provides enhanced data integrity necessary for data backup/restore operations.
In a recording operation, the tape drive emulator, in response to tape drive-based commands from the host computer, prepares a set of data for recording in a predetermined plural number of sectors on the disk, the set of data having set error correction bytes generated over the set, the error correction bytes being sufficient to recover one or more sectors of the disk not readable by the disk drive. Each set of data comprises a predetermined number of units.
In a first mode of the invention, a set of data is a frame comprising forty units, each unit being a disk sector. Each frame has packed and compressed logical blocks of data, along with sectors of ECC bytes generated over the frame. In addition, each frame has header bytes and CRC bytes. The frame error correction bytes of the first mode facilitate recovery of one or two unreadable sectors of a frame obtained from the disk. For example, if one or two sectors comprising a frame are unreadable from the disk, the frame error correction bytes enable the tape drive emulator to recover the lost sector(s), thereby providing a higher degree of data quality integrity.
In a second mode of the invention, a set of data is a super frame comprising forty units, each unit being akin to the forty-sector frame of the first embodiment. A super frame has a first predetermined number of user data frames and a second predetermined number of ECC frames. The ECC frames include ECC bytes generated over the entire super frame. A greater number of unreadable sectors are potentially recoverable using the second mode of the invention.
The present invention also includes a method for enabling a disk drive to detect whether a disk inserted therein has the set-based enhanced formatting of the present invention. Disks having the multi-sectored setbased enhanced formatting of the present invention are readable by enhanced drives, e.g., those with the tape drive emulator of the invention.
The detection method stems from a secondary formatting operation of the invention. In accordance with the secondary formatting operation, a format type identification is recorded on the disk. The format type identification is interpretable by an enhanced drive as indicating that the disk is compatible with transducing a multi-sectored set of data. Preferably, the format type identification is recorded as one disk sector at a beginning logical block address of the disk, and forms part of a volume header frame. The volume header frame is followed by a directory comprising one or more directory frames.
A frame can be one of several frame types, including a volume header frame, a data frame, an ECC frame (for the super frame mode), or a directory frame. A volume header frame includes volume identification information and a partition table. Part of the volume identification information is a volume format ID field (e.g., format type identification) which identifies the disk as having been formatted for use with the present invention. Directory frames are employed to compose a directory on the disk, e.g., at the beginning of the disk. The directory includes a frame index entry for each data frame recorded on the disk. The frame index entries enable the tape drive emulator to locate certain tape drive-related delimiters on the disk, such as logical block number, file marks, and set marks.